195 lines
6.1 KiB
Python
195 lines
6.1 KiB
Python
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"""
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Support for prepared statements
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"""
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# Copyright (C) 2020 The Psycopg Team
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from enum import IntEnum, auto
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from typing import Iterator, Optional, Sequence, Tuple, TYPE_CHECKING
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from collections import OrderedDict
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from typing_extensions import TypeAlias
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from . import pq
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from ._compat import Deque
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from ._queries import PostgresQuery
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if TYPE_CHECKING:
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from .pq.abc import PGresult
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Key: TypeAlias = Tuple[bytes, Tuple[int, ...]]
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COMMAND_OK = pq.ExecStatus.COMMAND_OK
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TUPLES_OK = pq.ExecStatus.TUPLES_OK
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class Prepare(IntEnum):
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NO = auto()
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YES = auto()
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SHOULD = auto()
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class PrepareManager:
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# Number of times a query is executed before it is prepared.
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prepare_threshold: Optional[int] = 5
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# Maximum number of prepared statements on the connection.
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prepared_max: int = 100
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def __init__(self) -> None:
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# Map (query, types) to the number of times the query was seen.
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self._counts: OrderedDict[Key, int] = OrderedDict()
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# Map (query, types) to the name of the statement if prepared.
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self._names: OrderedDict[Key, bytes] = OrderedDict()
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# Counter to generate prepared statements names
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self._prepared_idx = 0
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self._maint_commands = Deque[bytes]()
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@staticmethod
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def key(query: PostgresQuery) -> Key:
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return (query.query, query.types)
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def get(
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self, query: PostgresQuery, prepare: Optional[bool] = None
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) -> Tuple[Prepare, bytes]:
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"""
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Check if a query is prepared, tell back whether to prepare it.
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"""
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if prepare is False or self.prepare_threshold is None:
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# The user doesn't want this query to be prepared
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return Prepare.NO, b""
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key = self.key(query)
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name = self._names.get(key)
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if name:
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# The query was already prepared in this session
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return Prepare.YES, name
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count = self._counts.get(key, 0)
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if count >= self.prepare_threshold or prepare:
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# The query has been executed enough times and needs to be prepared
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name = f"_pg3_{self._prepared_idx}".encode()
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self._prepared_idx += 1
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return Prepare.SHOULD, name
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else:
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# The query is not to be prepared yet
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return Prepare.NO, b""
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def _should_discard(self, prep: Prepare, results: Sequence["PGresult"]) -> bool:
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"""Check if we need to discard our entire state: it should happen on
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rollback or on dropping objects, because the same object may get
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recreated and postgres would fail internal lookups.
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"""
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if self._names or prep == Prepare.SHOULD:
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for result in results:
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if result.status != COMMAND_OK:
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continue
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cmdstat = result.command_status
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if cmdstat and (cmdstat.startswith(b"DROP ") or cmdstat == b"ROLLBACK"):
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return self.clear()
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return False
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@staticmethod
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def _check_results(results: Sequence["PGresult"]) -> bool:
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"""Return False if 'results' are invalid for prepared statement cache."""
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if len(results) != 1:
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# We cannot prepare a multiple statement
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return False
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status = results[0].status
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if COMMAND_OK != status != TUPLES_OK:
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# We don't prepare failed queries or other weird results
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return False
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return True
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def _rotate(self) -> None:
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"""Evict an old value from the cache.
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If it was prepared, deallocate it. Do it only once: if the cache was
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resized, deallocate gradually.
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"""
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if len(self._counts) > self.prepared_max:
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self._counts.popitem(last=False)
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if len(self._names) > self.prepared_max:
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name = self._names.popitem(last=False)[1]
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self._maint_commands.append(b"DEALLOCATE " + name)
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def maybe_add_to_cache(
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self, query: PostgresQuery, prep: Prepare, name: bytes
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) -> Optional[Key]:
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"""Handle 'query' for possible addition to the cache.
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If a new entry has been added, return its key. Return None otherwise
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(meaning the query is already in cache or cache is not enabled).
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"""
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# don't do anything if prepared statements are disabled
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if self.prepare_threshold is None:
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return None
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key = self.key(query)
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if key in self._counts:
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if prep is Prepare.SHOULD:
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del self._counts[key]
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self._names[key] = name
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else:
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self._counts[key] += 1
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self._counts.move_to_end(key)
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return None
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elif key in self._names:
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self._names.move_to_end(key)
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return None
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else:
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if prep is Prepare.SHOULD:
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self._names[key] = name
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else:
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self._counts[key] = 1
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return key
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def validate(
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self,
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key: Key,
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prep: Prepare,
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name: bytes,
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results: Sequence["PGresult"],
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) -> None:
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"""Validate cached entry with 'key' by checking query 'results'.
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Possibly record a command to perform maintenance on database side.
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"""
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if self._should_discard(prep, results):
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return
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if not self._check_results(results):
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self._names.pop(key, None)
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self._counts.pop(key, None)
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else:
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self._rotate()
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def clear(self) -> bool:
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"""Clear the cache of the maintenance commands.
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Clear the internal state and prepare a command to clear the state of
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the server.
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"""
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self._counts.clear()
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if self._names:
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self._names.clear()
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self._maint_commands.clear()
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self._maint_commands.append(b"DEALLOCATE ALL")
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return True
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else:
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return False
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def get_maintenance_commands(self) -> Iterator[bytes]:
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"""
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Iterate over the commands needed to align the server state to our state
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"""
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while self._maint_commands:
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yield self._maint_commands.popleft()
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